DESCRIPTION (provided by investigator): The introduction of the novel "atypical" antipsychotic medications (e.g., olanzapine, risperidone) has led to this class of medication becoming the most frequently used medication treatment for behavioral and psychiatric disorders associated with developmental disabilities. The atypical antipsychotics are purported to have both a lower risk of antipsychotic-induced side effects (e.g., dyskinesia) and selective beneficial effects for stereotypy and self-injury in persons with developmental disabilities. There have been few adequate tests to date of these assumptions in persons with developmental disabilities. In our previous work with typical antipsychotic medications, we validated a battery of instrumental tests for measuring antipsychotic- induced effects on motor control, and we characterized the dynamic nature of antipsychotic-induced effects on both aberrant behaviors and abnormal movements as a function of the addition, maintenance, and withdrawal of typical antipsychotic agents. In the proposed study, we will examine in persons with mental retardation the variety of potential antipsychotic-induced effects associated with atypical antipsychotic treatment (dsperidone and olanzapine) using behavioral observations, movement kinematic analyses, motor control tests and adaptive performance tasks. In the proposed study we will conduct an acute double-blind comparison of the atypical antipsychotics risperidone and olanzapine to each other and to placebo. A longterm follow up will also be conducted of the effects of the atypical antipsychotic treatment over a 1-year period in subjects who are clinical responders to acute treatment with either olanzapine or risperidone. The following questions will be addressed: (1) Does treatment with atypical antipsychotic medication significantly reduce the occurrence of stereotyped behaviors and abnormal movements? (2) How does atypical antipsychotic medication affect the movement dynamic properties of stereotyped behaviors and abnormal movements? (3) How does atypical antipsychotic medication affect the performance of adaptive, goaloriented actions? And (4) Are there differences between the short and long term effects of atypical antipsychotics on aberrant behavior, abnormal movements, and goal-oriented motor task performance?